The present invention relates to a method and device for measuring the height of an object whose surface has irregular reflectance.
A height measuring device 100 of a conventional type concerned with the present invention comprises, as shown in FIG. 1, a measuring board 110, a floodlight optical system 120 provided in the upper part of the measuring board 110, a light receiving optical system 130 provided off to the upper left of the measuring board 110 and a signal processing circuit 140. The measuring board 110 has an XY stage which is not shown in FIG. 1. An object 101 is placed for measurement on the XY stage of the measuring board 110. The floodlight optical system 120 serves to irradiate laser light onto the surface of the object 101 and comprises a laser unit 121, a beam expander 122 composed of a plurality of lenses, a floodlight lens 123 and a reflector 124. Laser light L irradiated from the laser unit 121 is magnified into a large beam diameter by transmitting the beam expander 122, and then its beam diameter is converged by transmitting the floodlight lens 123. Laser light L transmitted the floodlight lens 123 is reflected downwardly by the reflector 124 as shown in FIG. 1 and then irradiated on the surface of the object 101 with a beam of a fixed diameter. The light receiving optical system 130 is provided for receiving a part of laser light L reflected by the surface of the object 101 (herein after referred to as "a reflected light R"), and comprises a light receiving lens 131 and a photodetector of position detecting type 132 disposed at the image forming position of the light receiving lens 131. Reflected light R is condensed by transmitting the light receiving lens 131 and then received by the photodetector of position detecting type 132. The signal processing unit 140 electrically connected to the photodetector of position detecting type 132 of the light receiving optical system 130 is used for detecting a light receiving position of reflected light R from a light receiving signal transmitted from the photodetector of position detecting type 132, and for calculating the height of the object 101 from the detected light receiving position by using the principle of triangulation.
Next, the height measurement principle of the height measuring device 100 will be described with reference to FIG. 2. It is assumed that the photodetector of position detecting type 132 is provided with its light receiving surface to which a part of laser light L reflected by the surface of the measuring board 110 (hereinafter referred to as "a reference reflected light R.sub.o ") inputs in perpendicular, when laser light L is vertically irradiated to the measuring board 110 without placing the object 101 thereon. Then by representing a distance between light receiving positions of reflected light R and reference reflected light R.sub.o in the photodetector of position detecting type 132 as "d," an angle between the optical axes of laser light L and the light receiving lens 131 as ".theta.", and a scale factor of the light receiving lens 131 as "m," the height of the object 101 placed on the measuring board 110 is given by the following expression. EQU h=d/(m.times.sin(.theta.)) (1)
Since the light receiving position of reference reflected light R.sub.o is known beforehand, the height of the object 101 can be obtained by detecting the light receiving position of reflected light R in the photodetector of position detecting type 132 by means of the signal processing circuit 140.
However, with the height measuring method of the conventional height measuring device 100 of this type, there occurs a problem such as shown in FIG. 3. That is, when the reflectance of surface S of the object 101 is uniform, the intensity centroid and the beam center of reflected light R coincide, so that the light receiving position Q.sub.1 of reflected light R in the photodetector of position detecting type 132 can be obtained from the position of the intensity centroid of reflected light R in the photodetector. On the other hand, when the reflectance of surface S of the object 101 is not uniform, the intensity centroid and the beam center of reflected light R do not coincide. Therefore, when the light receiving position of reflected light R in the photodetector of position detecting type 132 is obtained from the position of the intensity centroid of reflected light R in the photodetector, the light receiving position thus obtained is displaced in proportion to a displacement between the intensity centroid and the beam center of reflected light R, for example, as shown in FIG. 3 by light receiving positions Q.sub.2 and Q.sub.3. To be concrete, when a beam diameter of laser light L is represented by "D," an angle between the beam center line of laser light L and the optical axis of the light receiving lens 131 by ".theta..sub.1 " (herein after referred to as "a light receiving angle"), and further when the position of the intensity centroid of reflected light R in the photodetector of position detecting type 132 deviates from the position of the beam center of same reflected light R by utmost D/2, measurement error .DELTA.h given by the following expression is generated. EQU .DELTA.h=D/(2.times.tan(.theta..sub.1)) (2)
For example, when laser light L has beam diameter D=20 .mu.m, light receiving angle .theta..sub.1 =30.degree., then measurement error .DELTA.h=17.3 .mu.m. In addition to the above, as factors causing measurement error .DELTA.h, there are other factors such as fluctuation of the outgoing angle of the laser 121 or the displacement of an incident beam itself due to deviation of a scanning surface of a scanner used for scanning of laser light L in a scanning system.
In these circumstances, since height h of the object 101 is obtained with the conventional height measurement device 100 by assuming the position of the intensity centroid of reflected light R as the light receiving position of reflected light R in the photodetector of position detecting type 132, there occurs a problem that a measurement error .DELTA.h is included in height h thus obtained due to such as the unevenness of the reflectance of surface S of the object 101 or the displacement of the incident beam.